Roof and
attic thermal performance exert a powerful influence on cooling energy
use in Florida homes. The Florida Power and Light Company and the
Florida Solar Energy Center instrumented six side-by-side Habitat
homes in Ft. Myers, Florida with identical floor plans and orientation,
R-19 ceiling insulation, but with different roofing systems designed
to reduce attic heat gain. A seventh house had an unvented attic with
insulation on the underside of the roof deck rather than the ceiling:

Building
thermal conditions and air conditioning power usage were obtained.
The attic temperature during the peak summer hour is 40 oF greater
than ambient air temperature in the control home while no greater
than ambient with highly reflective roofing systems. Light colored
shingles and terra cotta roofs show temperatures in between those
extremes.

Measurements
showed that the three white reflective roofs would reduce cooling
energy consumption by 18-26% and peak demand by 28-35%. The terra
cotta tile roofs and white shingles would produce cooling savings
of 3-9% and 3-5%, respectively, while the sealed attic construction
with an insulated roof deck would produce reductions of 6-11%.

Introduction

Traditional
architecture in hot climates has long recognized that light building
colors can reduce cooling loads (Langewiesche, 1950; Givoni, 1976).
A series of simulation and experimental studies have demonstrated
that building reflectance can significantly impact cooling needs (Givoni
and Hoffmann, 1968; Taha et al., 1988; Griggs and Shipp, 1988, and
Bansal et al., 1992). Full building field tests in Florida and California
using before-after experiments have examined the impact of reflective
roofing on air conditioning (AC) energy use. In Florida tests measured
AC electrical savings averaged 19% (7.7 kWh/Day) (Parker et al., 1998).
Even greater fractional savings have been reported for similar experiments
in California (Akbari, et al., 1992). Beyond roof reflectance, additional
research has shown that other approaches such as tile roofs and unvented
attics with insulation under the roof itself, can produce cooler attics
resulting in energy improvements, but of unknown comparative magnitude
(Beal and Chandra, 1995; Rudd, 1998; Rudd et al., 2000).

Duct systems
are often located in the attic space in Sun Belt homes with slab on
grade foundations. In an early assessment of the impact of reflective
roofing, infrared thermo-graphy revealed that heat gain to attic-mounted
duct systems and air handlers are adversely affected by hot attics
(Parker et al, 1993B). As shown in Figure 1, previous analysis has
shown that attic heat gain to the thermal distribution system can
increase residential cooling loads by up to 30% during peak summer
periods (Parker et al., 1993; Jump et al, 1996). Further benefits
arise from the reduction of attic air temperature and its impact on
ceiling insulation conductivity (Levinson et al., 1996).

While previous
research efforts have investigated the thermal performance of various
roofing systems, this particular study represents the first time an
attempt has been made to quantify roofing influence on cooling performance
on identical, unoccupied, side-by-side residences. The project consisted
of seven, single-family residential homes located in Ft Meyers, Florida.
The focus of the study was to investigate how various roofing systems
impact air conditioning electrical demand. All seven residences had
a three bedroom, one bath floor plan and were of identical construction
and exposure. The houses underwent a series of tests in order to ensure
that the construction and mechanical systems performed similarly.
The sites were given a three-letter code to describe each roofing
system:

Monitoring
collected 15-minute data on comparative performance of the seven homes
in the summer of 2000 under unoccupied and carefully controlled conditions
for a month. Relevant construction details are summarized in Table
1.

The seventh
house (RSL) tested a new approach to residential insulation: an attic
completely sealed and with a spray foam insulation applied to the
underside of the roof decking in place of conventional blown or batt
insulation. The scheme insulates at the roof decking rather than at
the surface of the living space ceiling. Two primary advantages are
significantly less duct heat load within the attic space as well as
reduced humidity and infiltration. Research has shown this as a promising
construction technique (Rudd and Lstiburek, 1998; et al., 2000) in
a series of production homes in Las Vegas, Nevada and Tucson, Arizona.

One potential
disadvantage is that the roof insulation can result in significantly
higher decking and roof surface temperatures. Also, the insulation
at the roof deck has a more difficult task since it is working against
170 o (temperature of roofing) rather than 130 o (temperature on top
of insulation in a conventional attic at summer peak). The ducts are
exposed to less heat gain, but building heat transfer surface areas
are increased relative to the conventional case.

The roofing
system on the RSL home was identical to that in the control home,
dark gray composition shingles over roofing felt and decking. The
external appearance was like the conventional homes, however foam
insulation was used in the roof deck rather than cellulose insulation
in the ceiling assembly. The attic floor consisted solely of rafter
and ½ inch gypsum board. The roof deck of the RSL was covered
with 5 inches of insulating foam. Application thickness was targeted
to achieve an R-19 application – similar in thermal resistance
to the cellulose insulation in the other homes. The installed product
is a semi-rigid polyurethane foam insulation with a nominal density
of 0.45 - 0.5 lbs/ft 3 and an R-value of 3.81 ft 2-hr- oF/Btu/inch.
The product also claims to help improve air sealing of the home by
controlling leakage from building joints.

Calibrating
Thermostats and Influence of Set Temperature

Since variation
in interior thermostat temperature was known to be a large variable
controlling differences in space cooling, special effort was made
to carefully adjust the thermostats in each home so that each was
closely maintaining the same interior temperature. This was done using
thermocouples which measured the temperature in a central hallway
by the thermostat, but not overly close to it due to the heat emitted
from the electronics within the digital thermostat.

To evaluate
the impact of thermostat set temperature the thermostats were adjusted
up one
ºF for four days at the end of the project before the homes were occupied.
The typical increase was from 77
ºF to 78
ºF . These data were used to examine how the thermostat set-up influenced
cooling in order to properly adjust project results. This was accomplished
by searching for days in the set-up period having similar weather conditions.
As expected the impact was greatest on the cooler days where the outdoor temperature
approached the thermostat set temperature and solar radiation impacts were
minimized. Over the comparison, space cooling decreased by an average of 12.1%
per
ºF that the temperature was increased. When confined only to the peak
day, the impact was 8.3% per
ºF .

Results
over the Monitoring Period

The relative
performance of the homes over the entire unoccupied monitoring period
was evaluated. The five figures below (Figures 2-6) show the fundamental
impacts of the roofing system on cooling energy consumption over the
entire unoccupied monitoring period from July 8 th - July 31 st, 2000.

Figure
2 depicts the ambient average air temperature and solar conditions
over the entire unoccupied period. Figures 3, 4 and 5 show the thermal
influences of the roofing system. The first plot graphs the average
roof surface temperature over the daily cycle. The second plot shows
the corresponding temperature at the underside of the roof decking
surface. Note that the roof surface temperature and decking temperature
are highest with the sealed attic construction since the insulation
under the decking forces much of the collected solar heat to migrate
back out through the shingles. On average the shingles reach a peak
temperature that is seven degrees hotter than standard construction.
However, decking temperatures run almost 20 oF hotter. The white roofing
systems (RWM, RWF and RWB) experience peak surface temperatures approximately
20 oF lower than darker shingles. The terra cotta barrel tile case
runs about 10 o cooler.

The measured
mid attic air temperatures (Figure 5) above the ceiling insulation
revealed the impact of white reflective roofs with average peak temperatures
approximately 20 o cooler than at the control home. Whereas the attic
in the control home reaches 110 oF on the typical day, the attics
with the highly reflective white roofing materials only rise to about
90
ºF . Figure 6 shows the clear relationship between peak daily air temperature
and attic temperature for the differing roofing systems.

Figure
2. Average Ambient Air Temperature
and Solar Irradiance over the Unoccupied Period

Figure
3. Average Roof Surface Temperature Profiles over the Unoccupied
Period

Figure
4. Average Roof Decking Surface Temperature
Profiles over the Unoccupied Period

Figure
5. Average Attic Air Temperature Profiles over the Unoccupied
Period

Figure
6. Relationship of Daily Peak Air to Peak Attic Temperature

As expected, the home with the sealed attic had the lowest attic
temperatures reaching a maximum of 83 oF compared with the
77 oF being maintained inside. However, the sealed attic case
has no insulation on the ceiling floor with only studs and
sheet rock. Thus, from a cooling loads perspective, the low
attic temperature with this construction is deceptive. Since ½ inch
sheet rock only has a thermal resistance of 0.45 hr-ft 2- oF/Btu,
a significant level of heat transfer takes place across the
uninsulated ceiling. While this construction method reduced
attic air temperatures, it did not reduce ceiling heat transfer
as well as other options. Ceiling heat fluxes are actually
higher. In this case, the ceiling and duct system is unintentionally
cooling the attic space which can lead to the false impression
that roof/attic loads are lower.

Figure
7 summarizes the measured cooling load profiles for the seven homes
over the unoccupied monitoring period. Not surprisingly, the control
home has the highest consumption (17.0 kWh/day). The home with
the terra cotta barrel tile has a slightly lower use (16.2 kWh/day)
for a 5% cooling energy reduction. Next is the home with the white
shingles (15.6 kWh/day) – an 8% reduction. The sealed attic
comes in with a 12% cooling energy reduction (14.9 kWh/day).

Figure
7. Average Space Cooling Energy
Demand Profiles over the Unoccupied Period

The true white roofing types (> 60% reflectance) clearly show
their advantage. Both the white barrel and white flat tile roofs
averaged a consumption of 13.3 kWh/day or a 22% cooling energy reduction,
while the white metal roof shows the largest impact with a 12.2 kWh/day
August consumption for a 28% reduction. The numbers in Table 3 are
adjusted to account for differences in interior temperature and AC
performance:

It is
noteworthy that the average July temperature during the monitoring
period (81.6 oF) was very similar to the 30-year average for Ft.
Myers (82 oF). Thus, the current data are representative of typical
South Florida weather conditions. Relative to the standard control
home, the data show two distinct groups in terms of performance:

White
flat tile performed slightly better than the white barrel due to
its greater reflectance. The better performance of white metal
appears to come from the fact that lower nighttime and early morning
attic temperatures are achieved than those for tile or shingles,
leading to lower nighttime cooling demand.

Peak
Day Performance

July
26 th was one of the hottest and brightest days in the data collection
period and was used to evaluate peak influences. Average solar
irradiance was 371 W/m 2 and maximum temperature was 93.0
ºF. These data show that during periods of high solar insolation the performance
of the sealed attic case (RSL) suffers significantly. Decking and attic temperatures
are illustrated in Figures 8 and 9. For instance, both the tile roof and white
shingle did better at controlling demand than the sealed attic on this very
hot day. The white metal roof did best on the hottest day although not appreciably
different from the other white roofing types. Also, the savings for the white
roofs relative to the control were greater than for other days.

Table
4
Summer Peak Day Cooling Performance: July 26 th, 2000

Site

Cooling
Energy

Savings

Peak
Period*

kWh

Percent

Demand
(kW)

Savings(kW)

Percent

RGS

18.5
kWh

----

1.631

0.000

----

RWS

16.5
kWh

2.0

11%

1.439

0.192

11.8%

RSL

16.5
kWh

2.0

11%

1.626

0.005

0.3%

RTB

17.2
kWh

1.3

7%

1.570

0.061

3.7%

RWB

13.4
kWh

5.1

28%

1.073

0.558

34.2%

RWF

14.2
kWh

4.3

23%

1.019

0.612

37.5%

RWM

12.4
kWh

6.1

33%

0.984

0.647

39.7%

*
4-6 PM

Figure
8. Roof Decking Temperature Profiles for July 26, 2000

Figure
9. Attic Temperature Profiles for July 26, 2000

Analysis
of Energy Savings

We calculated
the annual cooling energy savings of the differing roofing materials
by two methods. First we estimated the normalized daily average
reduction in cooling kWh from each construction and then multiplied
this quantity by one over the fraction of average cooling which
takes place in the month of July. The normalized savings are the
values in Table 5 incorporating correction factors for air conditioner
performance and interior temperature differences.

The
fraction of space cooling in the month of July was obtained from
averaged empirical monitoring results from a large sample of homes
metered by the utility. These data show that 15.6% of total annual
cooling in the South region occurs in the month of July. Total
average cooling there for the month is 1,141 kWh or about 36.8
kWh/day. Since the homes in the Habitat study are only about 60%
of the average size of a typical new home, we suggest that savings
be indexed by the size of the ceiling against that in the study
(1,144 ft 2).

Table
5
Annual Cooling Energy Savings from Empirical Measurements

Site

Measured kWh/Day

Temp
Correction AC
Correction

kWh
Day Estimate

Savings (kWh/day)

Annual
Savings kWh*

RGS

17.03

1.000
/ 1.000

17.03

0.00

0

RWS

15.29

0.988
/ 1.092

16.49

0.54

110

RSL

14.73

1.054
/ 1.027

15.94

1.09

223

RTB

16.02

0.984
/ 0.978

15.42

1.60

329

RWB

13.32

1.021
/ 0.954

14.01

3.02

618

RWF

13.20

1.049
/ 1.014

12.86

4.17

852

RWM

12.03

1.028
/ 1.023

12.80

4.23

864

* The
estimate is based on a ratio of 6.59 for South (1/0.1517 for 15.17%
of cooling in July). House size assumptions must be accounted for
when estimating average savings or those for a specific case.

An alternative
calculation was made using daily regression results. The independent
parameters are the daily air temperature and average solar radiation
(global horizontal irradiance) that are used to estimate the daily
average kWh at that temperature and irradiance value. The regressions
for each home are then applied to Typical Meteorological Year (TMY)
data for Miami, Florida. The results shown in Table 6 become the
estimated space cooling use for each construction. Savings are
then normalized by the temperature, air conditioner performance
corrections and house size to yield final estimates.

Table
6
Annual Cooling Energy Savings from Regression Analysis Method

Site

kWh

kWh
Savings (*)

RGS

3679

0

RWS

3471

208
(191)

RSL

3242

437
(404)

RTB

3570

109
(113)

RWB

2809

870
(893)

RWF

2859

820
(771)

RWM

2584

1095
(1041)

(*)
Normalized to correct for off-reference temperature and AC performance.

Peak
Demand Reduction

Summer
peak demand savings were also estimated in two ways. First we used
the measured demand of the seven houses between the hours of 4
and 6 PM on the peak day (July 26, 2000). This estimate should
be indexed to the ceiling area for typical houses. Considering
that the typical Florida home likely has a ceiling area averaging
about 1770 square feet, the ratio of the impact would be approximately
55% greater than that estimated here (1,144 ft 2 ceiling). This
also better fits the average cooling energy demand from end-use
studies from the utility that show a summer peak AC demand of approximately
2.9 kW in occupied homes. Our monitoring study showed an average
peak demand of 1.63 kW in the control home. The average peak demand
for the study sites (based on the peak data for July 26 th) are
reproduced previously in Table 4.

The
second method of estimating demand reductions used hourly regression
equations. To estimate peak impacts, the regressions were evaluated
at an outdoor temperature of 92 oF – which is close to the
peak design temperature at 5 PM.

Simulation
Analysis

FSEC
has developed a hourly building energy software, EnergyGauge USA,
which runs based on the DOE-2.1E simulation engine. A key new component
in this software explicitly estimates the performance of attics
and the interactions of duct systems if located there. In Florida
homes, ducts are almost always in the attic space and previous
analysis shows that under peak conditions, the cooling system can
lose up to 30% of its cooling capacity through heat transfer with
the hot attic (Parker et al., 1993). The software has previously
been used to estimate the impact of reflective roofing around the
U.S. (Parker et al., 1998). It has been field validated in estimating
the space cooling energy use of three homes in Ocala, Florida.

We
created detailed input descriptions of each of the 1,114 ft² Habitat
homes in the study (including the shading impacts of surrounding
buildings) and simulated their performance to see how closely the
simulation could match the measured results. Table 8 shows the
results.

Table
8 Simulation
Analysis Results

Building
Site

Annual
Cooling(kWh)

Cooling
Savings
(kWh)

July
CoolingkWh
(kWh/D)

Peak
Cooling Demand

(kW)

Reduction

RGS

2,666

----

503
(16.2)

1.61

----

RWS

2,549

117

484
(15.6)

1.51

0.10

RSL

2,646

20

493
(15.9)

1.42

0.19

RTB

2,450

216

467
(15.1)

1.30

0.31

RWB

2,211

455

427
(13.8)

1.17

0.44

RWF

2,191

475

424
(13.7)

1.18

0.43

RWM

2,281

385

441
(14.2)

1.39

0.22

We noted
that the simulation predicts very similar absolute values to the
measured space cooling at the sites, although predicted savings
are somewhat less than those measured. Both the monitoring and
simulation shows the white roofing types to provide the greatest
savings and peak demand impact, followed by the tile roof, white
shingle and sealed attic construction.

Conclusions

Roof
and attic thermal performance exerts a powerful influence on cooling
energy use in Florida homes. Unshaded residential roofs are heated
by solar radiation during the daytime hours causing high attic
air temperatures. The large influence on cooling demand is due
both to the impact on ceiling heat transfer as well as heat gains
to the duct systems which are typically located in the attic space
with slab on grade construction.

With
the described project we tested six side-by-side Habitat homes
in South Florida with identical floor plans and orientations using
different roofing systems designed to reduce attic heat gains.
A seventh house with an unvented attic and insulated roof deck
was also included in the test.

The
data showed that solar heating had a large effect on attic thermal
performance in the control home. Air conditioning data were also
collected allowing characterization of the impact on cooling energy
use and peak electrical demand. Each of the examined alternative
roofing systems were found to be superior to standard dark shingles,
both in providing lower attic temperatures and lower AC energy
use. The sealed attic construction provided modest savings to cooling
energy, but no real peak reduction due to its sensitivity to periods
with high solar irradiance. Our research points to the need for
reflective roofing materials or light-colored tile roofing for
good energy performance with sealed attics.

In summary,
the selection of roof with high solar reflectance represents one
of the most significant energy-saving options available to homeowners
and home builders in hot climates. Further, the same materials
strongly reduce the house peak cooling demand during utility coincident
peak periods – a highly desirable attribute.

Acknowledgments

Special
thanks to Florida Power and Light Company who sponsored the study
and Habitat for Humanity who worked with FSEC to build the identical
test homes.